Electromeric effect

Electromeric effect refers to a molecular polarizability effect occurring by an intramolecular electron displacement (sometimes called the ‘conjugative mechanism’ and, previously, the ‘tautomeric mechanism’) characterized by the substitution of one electron pair for another within the same atomic octet of electrons.[1] The electromeric effect is often considered along with inductive effect as types of electron displacement.

This effect is shown by those compounds containing multiple bonds. When a double bond or triple bond is exposed to an attack by a reagent, a pair of bonding electrons involved in the pi bond is transferred completely from one atom to another. This effect will remain as long as the attacking reagent is present. As soon as the reagent is removed, the polarized molecule will come back to the original state.

+E and -E effects

Electromeric effect can be classified into +E and -E effects based on the direction of transfer of the electron pair.

+E effect is the breaking of non polar double bond or triple bond in the presence of attacking reagent as Electrophile.

-E effect is the breaking of polar double bond or triple bond in the presence of attacking reagent as Nucleophile.

In case of +E effect, the π electrons shift to an atom in order to form a cation and an anion on the atoms depending upon the stability of the individual species.Whereas in case of -E effect, the π electrons usually shift to the more electronegative atom.

Comparison between inductive effect and electromeric effect
Inductive Effect Electromeric Effect
The polarization of single σ covalent bond to the electronegativity difference between the bonding atoms is called inductive effect The complete shift of the π bond electron pair of a double bond or triple bond to one of the atoms joined by it in the presence of a suitable electrophilic reagent is called electromeric effect
It is a permanent effect It is a temporary effect
It doesn't need the presence of a reagent It needs the presence of an electrophilic or nucleophilic reagent

References
  1. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "electromeric effect". doi:10.1351/goldbook.E01973


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